1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus of transmitting a spatial stream for MU-MIMO in a wireless local area network system.
2. Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a WLAN is a technique which enables wireless access to the Internet at home or companies or in a specific service providing area using mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), on the basis of radio frequency technology.
Since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization of WLAN technology) has been set up on February, 1980, lots of standardization tasks are being performed.
The initial WLAN technology was able to support the bit rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication using a 2.4 GHz frequency band in accordance with IEEE 802.11, but the recent WLAN technology can support the maximum bit rate of 54 Mbps using Orthogonal Frequency Division Multiplex (OFDM). In addition, in the IEEE 802.11, the standardization of various techniques, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is put to practical use or being developed.
IEEE 802.11b of the IEEE 802.11 supports a maximum transmission speed of 11 Mbs while using the 2.4 GHz frequency band. IEEE 802.11a commercialized after the IEEE 802.11b has reduced the influence of interference as compared with the very complicated 2.4 GHz frequency band by using a 5 GHz frequency band not the 2.4 GHz frequency band and also improved the transmission speed up to a maximum of 54 Mbps using the OFDM technique. However, the IEEE 802.11a is disadvantageous in that the communication distance is shorter than that of the IEEE 802.11b. Further, IEEE 802.11g implements a maximum communication speed of 54 Mbps using the 2.4 GHz frequency band like the IEEE 802.11b and satisfies backward compatibility. The IEEE 802.11g is being in the spotlight and superior to the IEEE 802.11a even in the communication distance.
Further, as a technique for overcoming limits to the communication speed pointed out as vulnerabilities in the WLAN, there is IEEE 802.11n which has recently been standardized. The IEEE 802.11n has its object to increase the speed and reliability of a network and to expand the operating distance of a wireless network. More particularly, the IEEE 802.11n is based on a Multiple Inputs and Multiple Outputs (MIMO) technique using multiple antennas on both sides of a transmitter and a receiver in order to support a High Throughput (HT) having a data processing speed of 540 Mbps or higher, minimize transmission error, and optimize the data rate. Further, the IEEE 802.11n may use not only a coding method of transmitting several redundant copies in order to increase data reliability, but also an OFDM (Orthogonal Frequency Division Multiplex) method in order to increase the data rate.
With the WLAN being widely spread and applications using the WLAN becoming diverse, a need for a new WLAN system capable of supporting a higher throughput than the data processing speed supported by the IEEE 802.11n is recently gathering strength. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been proposed in order to support a data processing speed of 1 Gbps or higher. The name ‘VHT WLAN system’ is arbitrary. A feasibility test for a system using 8×8 MIMO and a channel bandwidth of 80 MHz or higher so as to provide the throughput of 1 Gbps or higher is in progress.
When implemented is a method for transmitting data to multiple STAs belonging to 802.11ac VHT WLAN system supporting the MU-MIMO transmission, the STAs must be informed through the VHT-SIG part of PLOP preamble that which STA receives data through which spatial stream. However, the association ID intended for identifying each individual STA requires a considerable number of bits; accordingly, a large amount of bits are required to inform the multiple STAs of spatial stream information. Therefore, one should take account of a method for reducing the number of bits carried by the PLOP preamble and informing STAs of the number of spatial streams.